In this study, we discuss the following two research themes on human discovery processes, using a traditional discovery task, the Wason's 2-4-6 task. One theme is the relation between nature of targets that subjects are required to find and an effective hypothesis testing strategy. The other theme is effects of collaboration in the process of discovery. The experimental results are summarized as follows. (1) There was interaction between targets' generality and an effective hypothesis testing strategy. That is, a positive test was effective in finding a specific target, while a negative test was effective in finding a general target. (2) The effect of two subjects collaboratively finding a target emerged only in a particular situation. That is, we confirmed, through our experiments, an evidnece of the effect of collaborative discovery when a pair of two subjects repeatedly conducted a positive test in their experiments for finding a general target.